import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int col = in.nextInt();
        int row = in.nextInt();
        int count = 0;
        int[][] array = new int[row][col];
        for (int i = 0; i < row; i++) {
            for(int j = 0; j < col; j++){
                if(array[i][j] == 0){
                    count++;
                    if(i+2 < row){
                        array[i+2][j] = 1;
                    }
                    if(j+2 < col){
                        array[i][j+2] = 1;
                    }
                }
            }
        }
        System.out.print(count);
    }
}